Imagine you have a baby kitten and a baby puppy. Now imagine that you give the baby kitten to the puppy mommy and the baby puppy to the kitten mommy. That's called cross-fostering. It means that one mommy animal takes care of a baby animal that isn't usually their own.
Sometimes people cross-foster baby animals for scientific reasons. They might want to see how the baby animals act and grow up differently depending on who their mommy is. Or they might want to study how different types of mommy animals take care of their babies.
Cross-fostering can also happen when a mommy animal can't take care of her babies for some reason. Maybe she's sick, or there are too many babies for her to take care of alone. When that happens, a human might take the babies and give them to a mommy animal who can take care of them better.
Cross-fostering can be a good way to make sure that every baby animal has a good mommy to take care of them, even if they weren't born to that mommy originally.